Many of the transmission paths between telephone central offices in larger cities in this country are by way of multipair cable. A multipair cable typically consists of 24 and 26 gauge pulp-insulated conductors for voice frequency paths, and 22-gauge pulp-insulated conductors for carrier circuits.
Pulp-insulated conductor cable has been the industry's standard for such short-haul routes because of its relatively low cost and its close packing. It provides an optimally large number of conductor pairs per unit cross-section of cable because of the relatively low dielectric constant of pulpous insulation. Also, there is continuing availability of the pulpous material as opposed to the increasing dependence on foreign sources for a petroleum derivative from which plastic insulating materials are made. A further advantage of pulp insulation is its ability to absorb moisture which localizes water at a fault point and facilitates the use of routine electrical tests to accurately locate the fault. A detailed description of a pulp-insulating process can be had by referring, for example, to an article "Manufacturing Pulp Cable", on pages 86-94 of the July-October 1971 issue of The Western Electric Engineer.
A pulp-insulated cable may contain as many as several thousand pairs of conductors. A conductor is covered with pulp insulation, and two insulated conductors are twisted into a pair. Pairs are then grouped into units, and units are assembled into cores. The cores are sheathed, for example, with a paper wrap and an overlay of aluminum, then steel, a flooding compound, and an outer plastic jacket. A typical prior art 3,600 pair cable of 26-gauge copper conductor has an outside diameter of 8.64 cm and a 3,000 pair cable, 7.59 cm.
With the increase in telephone traffic, a need has arisen to increase the capacity of short-haul, underground pulp-insulated cables. In some cities there is very little space for new ducts for the installation of new underground cable. Certain utilities, which have priority over telephone cables, occupy the portion of the underground space which is nearest street level. The result is that telephone cables are placed beneath other utility lines and in certain instances have extended well below street level. When it is necessary to add telephone cables, it is very difficult and expensive or impossible to locate new ducts below the existing ones.
Inasmuch as the installation of additional ducts is not attractive as a solution to the problem of increased underground capacity and the duct limits the diameter of the cable that can be installed, a new approach becomes necessary. Thought has been given to the use of a cable having an increased pair density of pulp-insulated conductors which could be used in existing ducts. Such a cable would require the use of less metal overlay material, less flooding compound, and less jacket material.
In order to provide a higher pair density pulp cable to reduce cable diameter for a given number of pairs or to provide a greater number of pulp-insulated conductors in an existing duct, it should be apparent that the outer diameter of each insulated conductor must be reduced. This has not been done because of the occurrence of uninsulated areas along the presently made conductors. These occur either because of a lack of adherence of the pulpous material to the conductors during insulating and because of the abuse to which the insulation is subjected in steps of a cable-making process subsequent to insulating.
The frequency of uninsulated areas along the conductor has been reduced by increasing the thickness of the pulp insulation. In fact, it also has been customary to provide spare pairs of conductors to supplant conductors which have such defects and are unusable for telecommunications. These solutions are counter-productive to today's goals because they increase the size of the cable cross-section and necessitate additional plastic jacketing material without an accompanying benefit in capacity.
Recently, a pulp-insulated conductor which has substantial mechanical integrity over that of prior art pulp conductors was introduced to the art. It is disclosed in U.S. Pat. No. 4,218,285 which issued on Aug. 19, 1980, in the names of H. E. Durr et al. Briefly, a metallic conductor is enclosed in a composite insulation cover comprising an adhesive coating and pulpous material. The adhesive coating bonds the pulpous material to the conductor.
The above-identified Durr et al arrangement provides consistent adhesion of the pulp-insulation to the conductor and provides improved mechanical properties. However, the dielectric constant of the adhesive coating between the conductor and the pulpous material is greater than the dielectric constant of the pulpous material. Undesirably, this causes the dielectric constant of the composite insulation to be increased.
The dielectric constant is a property of the conductor insulation which affects the transmission characteristics of a cable. For example, the mutual capacitance, Cm, is that capacitance characteristic in a telephone cable which affects transmission characteristics, is a function of the dielectric constant, and is inversely proportional to the center-to-center distance between conductors of each pair. The mutual capacitance of a cable is the average of the mutual capacitances of the pairs. (See F. W. Horn, ABC of the Telephone Cable, Inside and Out, pp. 1-3, Vol. 5).
Seemingly, the dielectric properties of the composite insulation and hence the capacitance characteristics of the cable would deteriorate still further if the thickness of the pulp insulation were to be reduced in order to provide an increased number of conductors in a given core size. Therefore, any increase in pair density through a reduction in the outer diameter of adhesive-coated pulp-insulated conductors must be accomplished within the framework of a suitable dielectric constant of the composite insulation and an acceptable mutual capacitance characteristic.
Another problem with respect to maximizing the use of existing underground ducts is the size of pulling eyes that are attached to cable ends and that are used to pull the cables through the ducts. The outside diameter of the cable pulling eyes generally has been greater than that of the cables to which the eyes are attached. As a result, additional space which otherwise could accommodate additional cable capacity remains within the duct about the cable.
A pulling eye having an outer diameter equal to that of the cable is shown in H. E. Durr U.S. Pat. No. 4,183,692 which issued on Jan. 15, 1980, which is incorporated by reference hereinto. While enough added pairs might be added to serve present needs by using this pulling eye, the solution would only be temporary and additional capacity most likely would be required in the not too distant future.
What is required is a cable system having an increased pulp-insulated conductor pair density which is capable of being received in existing underground ducts and which has acceptable mutual capacitance characteristics. The prior art has addressed part of this problem, and in U.S. Pat. No. 4,113,534 methods are disclosed for making pulp-insulated conductors with specific pulp fiber lengths and freeness ratios as well as specific process parameters.